Conventionally, it has been known to provide spark plugs with reduced size while having improved anti-fouling properties, such as a spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. 2002-260917 (“Patent Document 1”). This technique realizes a miniaturization of a spark plug as well as improving anti-fouling properties by way of reducing a clearance between a metal shell and an insulator located near a firing end of the spark plug.
In the thus-miniaturized spark plug, since the insulator also has a smaller diameter, improvement in breakage resistance thereof has been an issue. In particular, strength improvement has been required in a contact portion of a packing for securing airtightness and the insulator.
Such demand has been common not only with a spark plug having a small clearance between the metal shell and the insulator, but also with general spark plugs.
See also, Japanese Patent Application Laid-Open (kokai) No. 2005-183177 (“Patent Document 2”).